The association of the lac repressor with its operator is the best understood example of a specific protein-DNA recognition process. It is now clear that the conformations of both the repressor and the DNA are changed by this association. Quite likely, these conformational changes occur within the repressor-DNA complex, simultaneously altering the repressor and DNA in a manner which strengthens their interaction. We have attached fluorescent probes to the repressor, allowing us to view the dynamics of these conformational changes with stopped-flow rapid kinetic techniques. The association of the repressor with the nonoperator DNA poly(d(A-T); has been studied in detail. At least two conformational changes take place in addition to bimolecular association. These studies will be repeated using lac operator DNA. By comparing these two processes, we hope to reveal features essential for specific operator binding. Studies will also be undertaken to reveal whether these conformational changes precede or follow the association event, and if they affect all subunits or only those directly contacting the DNA. This information will allow us to propose detailed kinetic models of this interaction. These fluorescent probes should also prove useful in studying certain aspects of the conformational changes brought about by inducer.